1. Field of the Invention
Embodiments of the present invention relate generally to the field of betavoltaics; more particularly to a semiconductor-betavoltaic apparatus, method of fabrication, and applications thereof; and, more particularly to a silicon carbide (SiC) betavoltaic apparatus, method of fabrication, and applications thereof.
2. Technical Background
A betavoltaic battery consists of a semiconductor diode that is exposed to electrons emitted from a beta-emitting radioisotope thin film. The electrons penetrate the semiconductor material and generate electron-hole pairs by different ionization processes, which are collected across a built-in depletion layer electric field leading to current output with net power. Since the electrons are absorbed within a small absorption depth of only a few microns, a sufficiently large surface area of the exposed semiconductors is required, while maintaining high collection efficiency, to achieve high output electrical energy densities.
With very high energy densities of 1-10 mJ/cc (compared to 1-20 kJ/cc for conventional electrochemical and hydrocarbon fuels), and a long half-life of 1-100 years, radioisotope fueled batteries are ideal for applications requiring compact, long lifetime power supplies, such as remote sensing and implantable devices. Furthermore, low energy β emitters (63Ni, 147Pm, 3H, etc.) have little or no safety concerns, and Promethium-147 powered betavoltaics have been implanted inside humans for powering cardiac pacemakers in the past.
To achieve compact radioisotope batteries, the power density of the device should be as high as possible. The power output density of a betavoltaic battery can be expressed as follows:Pout=PfuelFFFηfuelηβ  (1)where Pfuel is the fuel power density, FFF is the fuel fill factor (volume percentage of the radioisotope fuel), ηfuel is radioisotope thin-film emission efficiency, and ηβ is betavoltaic conversion efficiency. Pfuel and ηfuel are determined by the radioisotope material. Higher energy β-emitting isotopes such as 137Cs and 90Sr have higher fuel power densities due to their high energy, but because these fuels emit very high electrons and significant x-ray flux, packaging volume increases significantly as shielding is needed, which decreases the overall power density of the battery. 63Ni emits β-particles with an average kinetic energy of 17.3 keV, with a penetration depth of less than 10 μm in most solids. As a result, devices powered by 63Ni thin-films, for example, can be deployed safely with millimeter or even microscale shields.
Different techniques of improving the FFF of a betavoltaic battery by patterning and etching of its active device layers have been previously reported; however, in all reported cases, the leakage currents were significantly increased due to the damage to the semiconductor materials in the etching process. Hence very low conversion efficiencies have been experimentally reported, and overall power density has seen little or no improvement in actual devices made so far.
The thicknesses of commercially available semiconductor (including but not limited to SiC and Si) wafers typically range from around 150 μm to 500 μm, where only the top approximately 20 μm is the active functioning region for a betavoltaic battery. Therefore, conventional planar betavoltaics may waste over 90% of their volume. Furthermore, in a planar device 50% of all of the electrons irradiated away from substrate are wasted.
The inventors have recognized the advantages and benefits of a betavoltaic device and associated fabrication methods that overcome the shortcomings and disadvantages recited above, as well as others known in the art.